1. Field of the Invention
This invention relates generally to tuned filters. More particularly, it relates to a tuning method and apparatus which provides high absolute accuracy in an RC time constant of an integrated circuit filter.
2. Background of Related Art
It is known that process variation in the tolerance of the absolute value of individual components formed in an integrated circuit (e.g., resistors and/or capacitors) can be quite great, but that similar components can be matched quite closely in value because it is likely that both will be affected equally by process and/or temperature variations. Thus, individual integrated circuit resistors can typically be manufactured to have a value only within a large range or tolerance.
One method to overcome this problem is the use of matched components. However, in certain applications such as where the absolute value of an RC time constant is important, the tolerance in the resistance and/or capacitance value providing the RC time constant dictates the accuracy of any filter basing its operation thereon. The accuracy of an RC time constant and thus a filter based thereon can be improved significantly by in-circuit tuning of the resistance forming the RC time constant to compensate for fluctuations in process and/or temperature.
A continuous time filter is a traditional technique, useful for removing high frequency out-of-band power of a signal (e.g., above 2 times the Nyquist rate). A continuous time filter is particularly useful in a circuit including sampling by an analog-to-digital (A/D) converter to provide higher accuracy in the samples. The present inventors desired to provide a more accurately tuned continuous time filter than those presently available using resistor based tuning.
Some applications, e.g., a central office codec used to digitize telephone speech, will use a switched capacitor filter switched at a high speed, e.g., 1 megahertz (MHz) to achieve a desired band pass (e.g., 4 kilohertz (kHz) band pass) together with a continuous time filter (e.g., a smoothing filter) having a wide tolerance due to the large tolerance of certain components such as resistors and/or capacitors forming an RC time constant therein. Such applications typically use both a band pass filter and a continuous time filter having low tolerance requirements, e.g., in the neighborhood of 35 kHz.
FIGS. 6 to 10 show a conventional technique for tuning the resistive portion of an RC time constant in a continuous time filter based on the variances of formed resistors only.
In particular, FIG. 6 shows a block diagram of an embodiment of a conventionally tuned continuous time filter operating on an input signal 410 to provide an output signal 420. A filter 600, e.g., a continuous time filter, filters an input signal 410 to provide an appropriately filtered output signal 420. The response of the filter 600 is controlled by one or more RC time constant(s). To provide the desired accuracy, the filter 600 is tuned by hand with a trimmed current source resistor tuning control circuit 400.
FIG. 7 is a schematic depiction of an embodiment of the conventionally tuned filter 600 shown in FIG. 6.
In particular, the trimmed current source resistor tuning control circuit 400 includes a trimmed current portion 502, and a comparators and latches portion 520. A relevant segment of the filter 600 is also shown in FIG. 7, as are the input signal 410 and the output signal 420.
The trimmed current portion 502 of the trimmed current source resistor tuning control circuit 400 includes a current mirror formed by two p-channel metal oxide semiconductor field effect transistors (PMOSFETs) 532, 534. A first side of the current mirror is trimmed with a current trimming element 536 (e.g., a variable resistor) to provide a desired current through the MOSFET 532. The current value set in this first side of the current mirror is then duplicated in the other side of the current mirror circuit, i.e., through MOSFET 534. The duplicated current is driven through a plurality of series connected resistors, e.g., four resistors 504, 506, 508 and 510.
Three comparators 522, 524 and 526 are fed on their respective positive inputs by nodes between each of the respective resistors 508 and 506, 506 and 504, and above resistor 504. The negative input of each of the three comparators 522, 524 and 526 is tied to a desired reference voltage VREF. The reference voltage VREF may be either internally generated on the integrated circuit or externally provided to the trimmed current source resistor tuning control circuit 400 from a source external to the integrated circuit.
The outputs of the comparators 522, 524 and 526 are respectively latched by latches 542, 544 and 546. The outputs of the comparators 522, 524 and 526 control the switching in or out of individual resistor in a tunable resistor element in the filter 600.
For example, tunable resistor components 613, 615 in the segment of the filter 600 shown in FIG. 7 are adjusted or `tuned` in accordance with the state of the outputs of the latches 542, 544 and 546. For instance, if the voltage reference VREF is at a level such that comparator 526 is saturated (i.e., the voltage level of the node between resistors 508 and 506 is greater than that of the voltage reference VREF), then latch 546 would have an active output thus turning on respective MOSFET switches 653a and 653b in the tunable filter 600, and accordingly short resistor 634 in the first resistive component 613 and resistor 644 in the balanced resistive component 615. Accordingly, the resistive components 613 and 615 (which are the balanced resistive portions of an RC time constant in the filter 600) are tuned within the allowable tolerance to the resistance of formed resistors 631-633 and 641-643, respectively, based on the performance of resistors 504-510 formed in the trimmed current portion 502.
Similarly, if the resistors 504-510 in the trimmed current portion 502 are such that a voltage level between resistors 506 and 504 exceeds that of the voltage reference VREF, then latches 544 and 546 have active outputs to cause closure of MOSFET switches 653a, 653b, 652a and 652b, to tune the resistive components 613 and 615 to the values of resistors 631-632 and 641-642, respectively. If the formed resistors 504-510 are such that the voltage level above the resistor 504 exceeds that of the voltage reference, then all three comparators 522-526 will become saturated and all three latches 542-546 will have active outputs when enabled, thus shorting all resistors except for resistor 631 in the first resistive element 613 and except for resistor 641 in the second resistive element 615.
For completeness, more detailed schematics of an embodiment of the conventional tunable continuous time filter shown in FIG. 6 are shown in FIGS. 8 to 10. In particular, FIG. 8 is a schematic diagram of the trimmed current portion 502 of the embodiment of the trimmed current source resistor tuning control circuit 400 shown in FIGS. 6 and 7. FIG. 9 is a schematic diagram of the comparators and latches portion 520 of the trimmed current source resistor tuning control circuit 400 shown in FIGS. 6 to 8. FIG. 10 is a schematic diagram of the filter 600 shown in FIG. 6.
Accordingly, FIGS. 6-10 show a conventional technique wherein a tunable resistor portion of an RC time constant in a filter 600 can be tuned based on formed resistors and a trimmed current. However, such conventional resistance-only based designs do not provide the desired precision or tolerance in the tuned filter, largely because variances in the other portion of the RC time constant, i.e., in the capacitive portions still leave a significant amount of error in the absolute value of the RC time constant. Moreover, many integrated circuit technologies do not allow for the manufacture of high density capacitors necessary for switched capacitor techniques.
Untuned filters in these technologies would likely have +/-50% tolerance. Even techniques based only on resistor tuning can reduce the variation to only approximately +/-25%.
Thus, there is a need for a tuning circuit for a continuous time filter which allows more accurate tuning of the RC time constant of a filter such that variances in both the resistor and the capacitor in the RC time constant are compensated for.